The overall objectives of this research project are to develop a methodology for an accurate comparison of venous return characteristics in dogs, swine, and humans. This comparison is designed to determine whether or not the parameters that regulate venous return in dogs and swine can be directly related to the same parameters in humans. Through painstaking analysis of human, canine, and swine venous return curves using the transient technique of altering Harvey oxygenator height, and by using bench and computer models, we have concluded that the transient technique cannot be used in humans for the characterization of the total systemic venous system. Our attention is now turned toward using a steady state technique during cardiopulmonary bypass to achieve our objective. Although the transient technique with the Harvey oxygenator cannot be used for measuring human venous parameters, the Harvey oxygenator can still be used to obtain vital venous system data in humans. If one assumes that the human venous system is a single compartment system, as suggested by Guyton, then a steady state measurement with the Harvey can be utilized. In this method, one makes a step change in blood flow rate and adjusts the level of the Harvey oxygenator such that the right atrial pressure remains constant. When a new steady state is achieved in the patients, the venous system time constant can be determined by comparing the new and old steady state blood volumes in the oxygenator (delta V/delta Q Pra equals tau). Separate measurements will then allow the determination of venous compliance and resistance. Since these measurements are taken between two steady states, the transient effects of holdup in the defoaming pads, holdup in the risers, feedback effects, and varying tube resistance are not relevant.